Re: optimization/9540: C99 initializers generate lots of code

Re: optimization/9540: C99 initializers generate lots of code

[Christian Ehrhardt]

The following reply was made to PR optimization/9540; it has been noted by GNATS.

From: "Christian Ehrhardt" <ehrhardt@mathematik.uni-ulm.de>
To: gcc-prs@gcc.gnu.org, mariube+gcc+@ifi.uio.no, gcc-bugs@gcc.gnu.org,
  gcc-gnats@gcc.gnu.org, nobody@gcc.gnu.org
Cc:  
Subject: Re: optimization/9540: C99 initializers generate lots of code
Date: Tue, 13 May 2003 15:17:39 +0200

 http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=9540
 
 The code size of the two functions differs because you do two
 fundamentally different things:
 
 * new_vector1 creates a compound literal (this is a separate object not
   just an initializer!) of type string. The contents of this object
   are then assigned to *result.
 * new_vector2 initializes *result directly without the construction of
   an intermediate compound literal.
 
 The additional instructions that you see are the result of the copying
 which is in general necessary because we can't know if gc_malloc might
 rely on the memory in *result or might modify previously initialized
 elements of the compound literal. The latter might be impossible
 due to aliasing rules but I'm not entirly sure about this. The former
 is definitely possible, e.g. if the first call to gc_malloc also stored
 the return value in some static or global variable.
 
 Given the fact that gcc does optimize the mem to mem copy away if
 the compound literal is declared const and gc_malloc is called outside
 of the initializer I guess that there aren't enough aliasing restrictions
 to allow the optimization otherwise.
 
    regards  Christian
 
 -- 
 THAT'S ALL FOLKS!

Re: optimization/9540: C99 initializers generate lots of code

[bangerth]

Synopsis: C99 initializers generate lots of code

State-Changed-From-To: feedback->open
State-Changed-By: bangerth
State-Changed-When: Mon Feb  3 21:02:25 2003
State-Changed-Why:
    Feedback received. The sizes of the two functions are
    indeed different, but I have too little knowledge of 
    the optimizers to analyze whether they differ in any
    measurable quantity (run-time, clock-ticks), so leave
    this to others.
    
    W.

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=9540